Topic 7 - Atomic and nuclear physics

Question 1

What is the nuclear model of atoms?

Answer 1

There is a small nucleus surrounded by electrons arranged into energy levels

Question 2

What is the problem with Rutherford’s model of the nuclear atom?

Answer 2

• Since electrons are charged, they should emit electromagnetic radiation all the time
• Their speed should decrease due to th loss of energy and the electron should move closer and closer to the nucleus in a spiral path until the atom collapses

Question 3

What does the Bohr model state?

Answer 3

Electrons in an atom can only have certain specific potential energies (electrons energy is quantized)

Question 4

What is the relationship between the speed of light, the frequency, and the wavelength of emitted electromagnetic radiation (light), when electrons fall from their excited state?

Answer 4

Question 5

What provides evidence for atomic energy levels?

Answer 5

Emission and absorption spectra

Question 6

Define nuclide

Answer 6

Atoms with the same nucleon number as well as proton number (isotopes)

Question 7

Distinguish between nuclide and isotope

Answer 7

When referring to the nuclei of different atoms → nuclide

When referring to the whole atom → isotope

Question 8

Define nucleon

Answer 8

The total number of protons and neutrons together in the nucleus = mass number A

Question 9

Describe the interactions in a nucleus

Answer 9

• Coulomb (repulsive) interactions between protons
• Strong, short-range nuclear interaction between nucleons pulling them together

Question 10

List the types of radiation

Answer 10

1. Alpha radiation
2. Beta radiation
3. Gamma radiation

Question 11

Describe alpha radiation

Answer 11

• Consists of a helium-4 nuclei
• Nuclei have a relative mass of four and a relative charge of +2
• Emission results in the loss of two protons and two neutrons from a nucleus
• The atomic number decreases by two and a new element is formed

Question 12

Describe beta radiation

Answer 12

• Consists of streams of high-velocity electrons moving at velocities close to the speed of light
• An electron is emitted from an unstable nucleus as the result of a neutron having converted into a proton
• The no. of nucleons stays the same but no. of protons increases by one, a new element is formed
• In addition to the beta particle, an additional antineutrino is released:

This is called beta-negative (B-) decay

Question 13

Describe gamma radiation

Answer 13

• High-energy electromagnetic radiation released from the nucleus
• Shorter wavelengths and higher energies than X-rays
• Long range and high penetrating power
• Often a by-product of beta or alpha
• Occurs when excited nuclei return to their ground state

Question 14

Describe the properties of alpha, beta, and gamma radiation

Answer 14

• Since alpha and beta are particles, they can be deflected by magnetic and electric fields
• Alpha particles have greater mass:charge ratio hence undergo less deflection
• Gamma radiation is uncharged and is not deflected

Question 15

Describe the ionising properties of alpha, beta, and gamma radiation

Answer 15

Alpha particles are large and massive so they are efficient ionisers. They collide with atoms and cause them to lose one or more electrons. Ion pairs are created. Since they lose their energy rapidly their penetrating power is low.

Beta particles have much more energy but smaller charge and mass so are less efficient ionisers but can penetrate better.

Gamma rays hardly ionise but penetrate matter very well.

Question 16

Outline the biological effects of ionising radiation

Answer 16

• Damages tissues of the body, more specifically cells
• Can cause cells to stop functioning (cell division) and may cause death
• Also long-term delayed effects such as sterility and cancers, and inherited mutations (DNA)
• Short term effects include radiation burns (beta and gamma), blindness, radiation sickness (nausea, fever, hair loss)
• Long-term effects of exposure to low doses remains unknown

Question 17

Explain why some nuclei are stable while others are unstable

Answer 17

There is a balance between electric force and nuclear force in the nucleus that needs to be maintained. Neutrons help to achieve this balance, since they increase the nucleus’ size.

The main factor that determines stability is the ratio of protons to neutrons. For light nuclei to be stable N = Z. For heavy nuclei to be stable N > Z. Heavy nuclei (Z > 80) are likely to alpha decay to reach more stable status.

Question 18

What is radioactive decay?

Answer 18

A random and spontaneous process, of which rate decreases exponentially with time

Question 19

Define radioactive half-life

Answer 19

The time taken for half the atoms in any given sample of a radioactive substance to decay and release their radiation.

Question 20

What is nuclear transmutation?

Answer 20

The transforming (changing) of one element/isotope into another element/isotope

Question 21

Why are artificial transmutations carried out?

Answer 21

Many artificial transmutations are carried out to prepare nuclides in medicine.

Question 22

Describe and give an example of an artificial (induced) transmutation

Answer 22

The nucleus of an atom is bombarded by neutrons, protons, deuterons, or alpha particles to change one element into another.

The kinetic energy needs to be high enough to overcome the Coulomb repulsion.

Question 23

Construct nuclear equations for alpha, beta, and gamma radiation

Answer 23

Question 24

Define unified atomic mass unit

Answer 24

One twelfth of the mass of a carbon-12 atom

u = 1.661 X 10^-27 kg

Question 25

What is Einstein’s mass-energy equivalence relationship?

Answer 25

It states that whenever a nuclear reaction results in the release of energy there is a corresponding decrease in mass:

Question 26

Define mass defect

Answer 26

The difference in mass between the mass of the nucleus and the masses of the individual protons and neutrons.

Related to the fact that in the process of combining the nucleon and the nucleus, the total mass decreases and energy is released.

Question 27

Define binding energy

Answer 27

The energy needed to separate a nucleus into its individual protons and neutrons (a negative quantity):

binding energy = mass defect X c²

Question 28

Define binding energy per nucleon

Answer 28

The average energy per nucleon needed to separate a nucleus into its individual nucleons:

Question 29

Draw and annotate a graph showing the variation with nucleon number of the binding energy per nucleon

Answer 29

Question 30

Define nuclear fission

Answer 30

The splitting of a heavy nucleus into two lighter nuclei accompanied by the release of large amounts of energy.

Question 31

Define nuclear fusion

Answer 31

The combination of two light nuclei to produce a heavier nucleus. Energy is released by the process.

Question 32

What is the main source of the Sun’s energy?

Answer 32

Nuclear fusion

Question 33

What provides evidence for the nuclear model of the atom?

Answer 33

Geiger-Marsden experiment:

• alpha particles were fired at a thin sheet of gold
• most of the alpha particles passed through the sheet
• a small number of particles deviated through an angle of more than 10°
• a smaller number of particles were deflected through an angle greater than 90°

Conclusions:

• most of the mass of the atom is concentrated in a very small volume at the centre of the atom
• the centre is positively charged in order to repel the alpha particles
• alpha particles that pass very close to the nucleus, ill experience electrostatic repulsion large enough to cause them to deviate through angles greater than 90°